It is known in the organ transplantation technique that the human body opposes transplantation of an organ/tissue by rejection phenomena caused by the immune defense of the body. In order to avoid immunosuppressive medicaments (secondary effects, impairment of the immune defense) bioartificial implants have been developed which comprise donor tissue/cells which are to be implanted and a semipermeable barrier or “filter” which is to allow diffusion of nutrients and oxygen from the donee's body to the implanted (i.e. transplanted) donor tissue/cells, but no diffusion of the donee's immune defense mechanisms (cells), while at the same time it shall allow diffusion of desirable substances produced by the donor tissue/cells to the donee's body. Examples in the patent literature concerning such bioartificial implants include U.S. Pat. No. 6,632,244 and WO 02/02745, which discuss particularly implantation of isolated islets of Langerhans (bio-artificial pancreas) for production of insulin in the donee's body. U.S. Pat. No. 6,632,244 also discusses the risk of fibrosis (growth of connective tissue) in the donee's body, which results in the pores of the barrier being covered by connective tissue, whereby the implant “starves to death” (hypoxia), since it does not receive oxygen and nutrients from the donee's body. This phenomenon occurs in barrier materials which are not biocompatible. According to the latter patent, this inconvenience is avoided with a biocompatible implant in the form of a thin sheet of three components (a) a core, which consists of living tissue, trophic factors and nurse cells, an alginate polymer crosslinked with, for instance, calcium, and a fiber net for strength, (b) a coating of alginate polymer crosslinked with calcium for control of permeability, and (c) a coating which also consists of crosslinked alginate polymer. It is recommended that the thickness of the implant not exceed 400 μm.
A transplant which is also coated with alginate (multilayer) is disclosed in U.S. Pat. No. 5,876,742. The coating is said to be non-fibrogenic. The thickness of the coating is 20-200 μm.
U.S. Pat. No. 5,782,912 discloses an implant with a wall consisting of a first porous membrane, which is proximal to the donee's tissue and which is said to favor formation of vascular structures in a donee membrane interface and there prevent formation of connective tissue. The implant also has a second porous membrane, which forms an immuno-isolated space. The space encloses tissue, pancreas islets, which must be protected from being contacted by the donee's cells. The second membrane allows diffusion of components which are generated by the enclosed tissue, producing insulin for instance. The second membrane also allows diffusion of nutrient from the donee to the space in order to provide the tissue with nutrient. The membranes are made of polymer, and the first membrane has a special three-dimensional pore structure. This patent publication states that “Known biocompatible medical implants are composed of ceramics and metals. Assuming these materials could be manipulated to provide the three-dimensional structures described herein, they would also be useful in the present invention”.
U.S. Pat. No. 5,782,912 also discloses use of the device as a coating on an indwelling sensor and on an indwelling catheter, as means for transport of physiological factors to indwelling sensors, as means for transport of drugs from a chamber or catheter to donee tissue and as means for encapsulation of grafted cells for treatment of cellular and molecular deficiencies (immunoisolation).
The above-described implants and other implants based on semipermeable barriers are complicated in design and difficult to produce. There is reason to believe that these circumstances do not result in well-reproducible implants with the desired properties. There is also reason to believe that the vascularising barrier of the known implants is not optimally efficient to prevent fibrosis since a liquid-filled gap between connective tissue and barrier is not taken into consideration.